<!DOCTYPE html>
<html lang="en-us">
  <head>

    <meta http-equiv="content-type" content="text/html; charset=utf-8">
    
<meta charset="UTF-8">
<title>Resilience in small clusters | Elasticsearch Guide [7.7] | Elastic</title>
<link rel="home" href="index.html" title="Elasticsearch Guide [7.7]">
<link rel="up" href="high-availability-cluster-design.html" title="Designing for resilience">
<link rel="prev" href="high-availability-cluster-design.html" title="Designing for resilience">
<link rel="next" href="high-availability-cluster-design-large-clusters.html" title="Resilience in larger clusters">
<meta name="DC.type" content="Learn/Docs/Elasticsearch/Reference/7.7">
<meta name="DC.subject" content="Elasticsearch">
<meta name="DC.identifier" content="7.7">
<meta name="robots" content="noindex,nofollow">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script src="https://cdn.optimizely.com/js/18132920325.js"></script>
    <link rel="apple-touch-icon" sizes="57x57" href="/apple-icon-57x57.png">
    <link rel="apple-touch-icon" sizes="60x60" href="/apple-icon-60x60.png">
    <link rel="apple-touch-icon" sizes="72x72" href="/apple-icon-72x72.png">
    <link rel="apple-touch-icon" sizes="76x76" href="/apple-icon-76x76.png">
    <link rel="apple-touch-icon" sizes="114x114" href="/apple-icon-114x114.png">
    <link rel="apple-touch-icon" sizes="120x120" href="/apple-icon-120x120.png">
    <link rel="apple-touch-icon" sizes="144x144" href="/apple-icon-144x144.png">
    <link rel="apple-touch-icon" sizes="152x152" href="/apple-icon-152x152.png">
    <link rel="apple-touch-icon" sizes="180x180" href="/apple-icon-180x180.png">
    <link rel="icon" type="image/png" href="/favicon-32x32.png" sizes="32x32">
    <link rel="icon" type="image/png" href="/android-chrome-192x192.png" sizes="192x192">
    <link rel="icon" type="image/png" href="/favicon-96x96.png" sizes="96x96">
    <link rel="icon" type="image/png" href="/favicon-16x16.png" sizes="16x16">
    <link rel="manifest" href="/manifest.json">
    <meta name="apple-mobile-web-app-title" content="Elastic">
    <meta name="application-name" content="Elastic">
    <meta name="msapplication-TileColor" content="#ffffff">
    <meta name="msapplication-TileImage" content="/mstile-144x144.png">
    <meta name="theme-color" content="#ffffff">
    <meta name="naver-site-verification" content="936882c1853b701b3cef3721758d80535413dbfd">
    <meta name="yandex-verification" content="d8a47e95d0972434">
    <meta name="localized" content="true">
    <meta name="st:robots" content="follow,index">
    <meta property="og:image" content="https://www.elastic.co/static/images/elastic-logo-200.png">
    <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon">
    <link rel="icon" href="/favicon.ico" type="image/x-icon">
    <link rel="apple-touch-icon-precomposed" sizes="64x64" href="/favicon_64x64_16bit.png">
    <link rel="apple-touch-icon-precomposed" sizes="32x32" href="/favicon_32x32.png">
    <link rel="apple-touch-icon-precomposed" sizes="16x16" href="/favicon_16x16.png">
    <!-- Give IE8 a fighting chance -->
    <!--[if lt IE 9]>
    <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
    <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
    <![endif]-->
    <link rel="stylesheet" type="text/css" href="/guide/static/styles.css">
  </head>

  <!--© 2015-2021 Elasticsearch B.V. Copying, publishing and/or distributing without written permission is strictly prohibited.-->

  <body>
    <!-- Google Tag Manager -->
    <script>dataLayer = [];</script><noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-58RLH5" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
    <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start': new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0], j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src= '//www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f); })(window,document,'script','dataLayer','GTM-58RLH5');</script>
    <!-- End Google Tag Manager -->

    <!-- Global site tag (gtag.js) - Google Analytics -->
    <script async src="https://www.googletagmanager.com/gtag/js?id=UA-12395217-16"></script>
    <script>
      window.dataLayer = window.dataLayer || [];
      function gtag(){dataLayer.push(arguments);}
      gtag('js', new Date());
      gtag('config', 'UA-12395217-16');
    </script>

    <!--BEGIN QUALTRICS WEBSITE FEEDBACK SNIPPET-->
    <script type="text/javascript">
      (function(){var g=function(e,h,f,g){
      this.get=function(a){for(var a=a+"=",c=document.cookie.split(";"),b=0,e=c.length;b<e;b++){for(var d=c[b];" "==d.charAt(0);)d=d.substring(1,d.length);if(0==d.indexOf(a))return d.substring(a.length,d.length)}return null};
      this.set=function(a,c){var b="",b=new Date;b.setTime(b.getTime()+6048E5);b="; expires="+b.toGMTString();document.cookie=a+"="+c+b+"; path=/; "};
      this.check=function(){var a=this.get(f);if(a)a=a.split(":");else if(100!=e)"v"==h&&(e=Math.random()>=e/100?0:100),a=[h,e,0],this.set(f,a.join(":"));else return!0;var c=a[1];if(100==c)return!0;switch(a[0]){case "v":return!1;case "r":return c=a[2]%Math.floor(100/c),a[2]++,this.set(f,a.join(":")),!c}return!0};
      this.go=function(){if(this.check()){var a=document.createElement("script");a.type="text/javascript";a.src=g;document.body&&document.body.appendChild(a)}};
      this.start=function(){var a=this;window.addEventListener?window.addEventListener("load",function(){a.go()},!1):window.attachEvent&&window.attachEvent("onload",function(){a.go()})}};
      try{(new g(100,"r","QSI_S_ZN_emkP0oSe9Qrn7kF","https://znemkp0ose9qrn7kf-elastic.siteintercept.qualtrics.com/WRSiteInterceptEngine/?Q_ZID=ZN_emkP0oSe9Qrn7kF")).start()}catch(i){}})();
    </script><div id="ZN_emkP0oSe9Qrn7kF"><!--DO NOT REMOVE-CONTENTS PLACED HERE--></div>
    <!--END WEBSITE FEEDBACK SNIPPET-->

    <div id="elastic-nav" style="display:none;"></div>
    <script src="https://www.elastic.co/elastic-nav.js"></script>

    <!-- Subnav -->
    <div>
      <div>
        <div class="tertiary-nav d-none d-md-block">
          <div class="container">
            <div class="p-t-b-15 d-flex justify-content-between nav-container">
              <div class="breadcrum-wrapper"><span><a href="/guide/" style="font-size: 14px; font-weight: 600; color: #000;">Docs</a></span></div>
            </div>
          </div>
        </div>
      </div>
    </div>

    <div class="main-container">
      <section id="content">
        <div class="content-wrapper">

          <section id="guide" lang="en">
            <div class="container">
              <div class="row">
                <div class="col-xs-12 col-sm-8 col-md-8 guide-section">
                  <!-- start body -->
                  <div class="page_header">
<strong>IMPORTANT</strong>: No additional bug fixes or documentation updates
will be released for this version. For the latest information, see the
<a href="../current/index.html">current release documentation</a>.
</div>
<div id="content">
<div class="breadcrumbs">
<span class="breadcrumb-link"><a href="index.html">Elasticsearch Guide [7.7]</a></span>
»
<span class="breadcrumb-link"><a href="high-availability.html">Set up a cluster for high availability</a></span>
»
<span class="breadcrumb-link"><a href="high-availability-cluster-design.html">Designing for resilience</a></span>
»
<span class="breadcrumb-node">Resilience in small clusters</span>
</div>
<div class="navheader">
<span class="prev">
<a href="high-availability-cluster-design.html">« Designing for resilience</a>
</span>
<span class="next">
<a href="high-availability-cluster-design-large-clusters.html">Resilience in larger clusters »</a>
</span>
</div>
<div class="section">
<div class="titlepage"><div><div>
<h2 class="title">
<a id="high-availability-cluster-small-clusters"></a>Resilience in small clusters<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h2>
</div></div></div>
<p>In smaller clusters, it is most important to be resilient to single-node
failures. This section gives some guidance on making your cluster as resilient
as possible to the failure of an individual node.</p>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-one-node"></a>One-node clusters<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>If your cluster consists of one node, that single node must do everything.
To accommodate this, Elasticsearch assigns nodes every role by default.</p>
<p>A single node cluster is not resilient. If the the node fails, the cluster will
stop working. Because there are no replicas in a one-node cluster, you cannot
store your data redundantly. However, by default at least one replica is
required for a <a class="xref" href="cluster-health.html" title="Cluster health API"><code class="literal">green</code> cluster health status</a>. To ensure your
cluster can report a <code class="literal">green</code> status, override the default by setting
<a class="xref" href="index-modules.html#dynamic-index-settings" title="Dynamic index settings"><code class="literal">index.number_of_replicas</code></a> to <code class="literal">0</code> on every index.</p>
<p>If the node fails, you may need to restore an older copy of any lost indices
from a <a class="xref" href="modules-snapshots.html" title="Snapshot module">snapshot</a>.</p>
<p>Because they are not resilient to any failures, we do not recommend using
one-node clusters in production.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-two-nodes"></a>Two-node clusters<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>If you have two nodes, we recommend they both be data nodes. You should also
ensure every shard is stored redundantly on both nodes by setting
<a class="xref" href="index-modules.html#dynamic-index-settings" title="Dynamic index settings"><code class="literal">index.number_of_replicas</code></a> to <code class="literal">1</code> on every index.
This is the default number of replicas but may be overridden by an
<a class="xref" href="indices-templates.html" title="Put index template API">index template</a>. <a class="xref" href="index-modules.html#dynamic-index-settings" title="Dynamic index settings">Auto-expand
replicas</a> can also achieve the same thing, but it’s not necessary to use this
feature in such a small cluster.</p>
<p>We recommend you set <code class="literal">node.master: false</code> on one of your two nodes so that it is
not <a class="xref" href="modules-node.html#master-node" title="Master-eligible node">master-eligible</a>. This means you can be certain which of your
nodes is the elected master of the cluster. The cluster can tolerate the loss of
the other master-ineligible node. If you don’t set <code class="literal">node.master: false</code> on one
node, both nodes are master-eligible. This means both nodes are required for a
master election. Since the election will fail if either node is unavailable,
your cluster cannot reliably tolerate the loss of either node.</p>
<p>By default, each node is assigned every role. We recommend you assign both nodes
all other roles except master eligibility. If one node fails, the other node can
handle its tasks.</p>
<p>You should avoid sending client requests to just one of your nodes. If you do
and this node fails, such requests will not receive responses even if the
remaining node is a healthy cluster on its own. Ideally, you should balance your
client requests across both nodes. A good way to do this is to specify the
addresses of both nodes when configuring the client to connect to your cluster.
Alternatively, you can use a resilient load balancer to balance client requests
across the nodes in your cluster.</p>
<p>Because it’s not resilient to failures, we do not recommend deploying a two-node
cluster in production.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-two-nodes-plus"></a>Two-node clusters with a tiebreaker<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Because master elections are majority-based, the two-node cluster described
above is tolerant to the loss of one of its nodes but not the
other one. You cannot configure a two-node cluster so that it can tolerate
the loss of <em>either</em> node because this is theoretically impossible. You might
expect that if either node fails then Elasticsearch can elect the remaining node as the
master, but it is impossible to tell the difference between the failure of a
remote node and a mere loss of connectivity between the nodes. If both nodes
were capable of running independent elections, a loss of connectivity would
lead to a <a href="https://en.wikipedia.org/wiki/Split-brain_(computing)" class="ulink" target="_top">split-brain
problem</a> and therefore data loss. Elasticsearch avoids this and
protects your data by electing neither node as master until that node can be
sure that it has the latest cluster state and that there is no other master in
the cluster. This could result in the cluster having no master until
connectivity is restored.</p>
<p>You can solve this problem by adding a third node and making all three nodes
master-eligible. A <a class="xref" href="modules-discovery-quorums.html" title="Quorum-based decision making">master election</a> requires only
two of the three master-eligible nodes. This means the cluster can tolerate the
loss of any single node. This third node acts as a tiebreaker in cases where the
two original nodes are disconnected from each other. You can reduce the resource
requirements of this extra node by making it a <a class="xref" href="modules-node.html#voting-only-node" title="Voting-only master-eligible node">dedicated
voting-only master-eligible node</a>, also known as a dedicated tiebreaker.
Because it has no other roles, a dedicated tiebreaker does not need to be as
powerful as the other two nodes. It will not perform any searches nor coordinate
any client requests and cannot be elected as the master of the cluster.</p>
<p>The two original nodes should not be voting-only master-eligible nodes since a
resilient cluster requires at least three master-eligible nodes, at least two
of which are not voting-only master-eligible nodes. If two of your three nodes
are voting-only master-eligible nodes then the elected master must be the third
node. This node then becomes a single point of failure.</p>
<p>We recommend assigning both non-tiebreaker nodes all other roles. This creates
redundancy by ensuring any task in the cluster can be handled by either node.</p>
<p>You should not send any client requests to the dedicated tiebreaker node.
You should also avoid sending client requests to just one of the other two
nodes. If you do, and this node fails, then any requests will not
receive responses, even if the remaining nodes form a healthy cluster. Ideally,
you should balance your client requests across both of the non-tiebreaker
nodes. You can do this by specifying the address of both nodes
when configuring your client to connect to your cluster. Alternatively, you can
use a resilient load balancer to balance client requests across the appropriate
nodes in your cluster. The <a href="/cloud/elasticsearch-service/signup?baymax=docs-body&amp;elektra=docs" class="ulink" target="_top">Elastic Cloud</a> service
provides such a load balancer.</p>
<p>A two-node cluster with an additional tiebreaker node is the smallest possible
cluster that is suitable for production deployments.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-three-nodes"></a>Three-node clusters<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>If you have three nodes, we recommend they all be <a class="xref" href="modules-node.html#data-node" title="Data node">data
nodes</a> and every index should have at least one replica. Nodes are data nodes
by default. You may prefer for some indices to have two replicas so that each
node has a copy of each shard in those indices. You should also configure each
node to be <a class="xref" href="modules-node.html#master-node" title="Master-eligible node">master-eligible</a> so that any two of them can hold a
master election without needing to communicate with the third node. Nodes are
master-eligible by default. This cluster will be resilient to the loss of any
single node.</p>
<p>You should avoid sending client requests to just one of your nodes. If you do,
and this node fails, then any requests will not receive responses even if the
remaining two nodes form a healthy cluster. Ideally, you should balance your
client requests across all three nodes. You can do this by specifying the
address of multiple nodes when configuring your client to connect to your
cluster. Alternatively you can use a resilient load balancer to balance client
requests across your cluster. The <a href="/cloud/elasticsearch-service/signup?baymax=docs-body&amp;elektra=docs" class="ulink" target="_top">Elastic Cloud</a>
service provides such a load balancer.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-three-plus-nodes"></a>Clusters with more than three nodes<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Once your cluster grows to more than three nodes, you can start to specialise
these nodes according to their responsibilities, allowing you to scale their
resources independently as needed. You can have as many <a class="xref" href="modules-node.html#data-node" title="Data node">data
nodes</a>, <a class="xref" href="ingest.html" title="Ingest node">ingest nodes</a>, <a class="xref" href="modules-node.html#ml-node" title="Machine learning node">machine learning nodes</a>, etc. as needed to
support your workload. As your cluster grows larger, we recommend using
dedicated nodes for each role. This lets you to independently scale resources
for each task.</p>
<p>However, it is good practice to limit the number of master-eligible nodes in
the cluster to three. Master nodes do not scale like other node types since
the cluster always elects just one of them as the master of the cluster. If
there are too many master-eligible nodes then master elections may take a
longer time to complete. In larger clusters, we recommend you
configure some of your nodes as dedicated master-eligible nodes and avoid
sending any client requests to these dedicated nodes. Your cluster may become
unstable if the master-eligible nodes are overwhelmed with unnecessary extra
work that could be handled by one of the other nodes.</p>
<p>You may configure one of your master-eligible nodes to be a
<a class="xref" href="modules-node.html#voting-only-node" title="Voting-only master-eligible node">voting-only node</a> so that it can never be elected as the
master node. For instance, you may have two dedicated master nodes and a third
node that is both a data node and a voting-only master-eligible node. This
third voting-only node will act as a tiebreaker in master elections but will
never become the master itself.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="high-availability-cluster-design-small-cluster-summary"></a>Summary<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/high-availability/cluster-design.asciidoc">edit</a>
</h3>
</div></div></div>
<p>The cluster will be resilient to the loss of any node as long as:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
The <a class="xref" href="cluster-health.html" title="Cluster health API">cluster health status</a> is <code class="literal">green</code>.
</li>
<li class="listitem">
There are at least two data nodes.
</li>
<li class="listitem">
Every index has at least one replica of each shard, in addition to the
primary.
</li>
<li class="listitem">
The cluster has at least three master-eligible nodes, as long as at least two
of these nodes are not voting-only master-eligible nodes.
</li>
<li class="listitem">
Clients are configured to send their requests to more than one node or are
configured to use a load balancer that balances the requests across an
appropriate set of nodes. The <a href="/cloud/elasticsearch-service/signup?baymax=docs-body&amp;elektra=docs" class="ulink" target="_top">Elastic Cloud</a> service provides such
a load balancer.
</li>
</ul>
</div>
</div>

</div>
<div class="navfooter">
<span class="prev">
<a href="high-availability-cluster-design.html">« Designing for resilience</a>
</span>
<span class="next">
<a href="high-availability-cluster-design-large-clusters.html">Resilience in larger clusters »</a>
</span>
</div>
</div>

                  <!-- end body -->
                </div>
                <div class="col-xs-12 col-sm-4 col-md-4" id="right_col">
                  <div id="rtpcontainer" style="display: block;">
                    <div class="mktg-promo">
                      <h3>Most Popular</h3>
                      <ul class="icons">
                        <li class="icon-elasticsearch-white"><a href="https://www.elastic.co/webinars/getting-started-elasticsearch?baymax=default&amp;elektra=docs&amp;storm=top-video">Get Started with Elasticsearch: Video</a></li>
                        <li class="icon-kibana-white"><a href="https://www.elastic.co/webinars/getting-started-kibana?baymax=default&amp;elektra=docs&amp;storm=top-video">Intro to Kibana: Video</a></li>
                        <li class="icon-logstash-white"><a href="https://www.elastic.co/webinars/introduction-elk-stack?baymax=default&amp;elektra=docs&amp;storm=top-video">ELK for Logs &amp; Metrics: Video</a></li>
                      </ul>
                    </div>
                  </div>
                </div>
              </div>
            </div>
          </section>

        </div>


<div id="elastic-footer"></div>
<script src="https://www.elastic.co/elastic-footer.js"></script>
<!-- Footer Section end-->

      </section>
    </div>

<script src="/guide/static/jquery.js"></script>
<script type="text/javascript" src="/guide/static/docs.js"></script>
<script type="text/javascript">
  window.initial_state = {}</script>
  </body>
</html>
